Hi I had the same problem but I nearly fixed it I have the new 8350 fx 4.0ghz processor and 2x8gb of 1866 ram and the mobo would downclock to 1600.
I fixed this by enabling some option in the AI tweaker called something like "OC Tuner" It will say "OK" as the option click it and it will do some sort of reset. when I rebooted I changed my RAM frequency to 1728MHZ. I rebooted and found that my processor had been OC'ed to 4.3ghz and the RAM was running at 1728MHZ, it's not 1866 but it's as close as I could get.
Hope this was helpful...
ASUS Sabertooth fx mobo
2x8gb 1866mhz ram
fx 8350 4.0/4.3ghz OC processor
860 watt PSU